Alfalfa Autotoxicity - Frequently Asked Questions

What is autotoxicity?
Plants produce many different chemicals that they use to defend themselves from things like insects and diseases. Certain plant species give off chemicals that affect the growth and development of other plants. This is called allelopathy. Alfalfa has an allelopathic chemical that inhibits the growth of other alfalfa plants. It is said to be autotoxic, or toxic to itself.

What causes autotoxicity in alfalfa?
The autotoxic chemicals produced by alfalfa are water soluble and leach into the soil from decomposing plant material and growing plants. The chemical causing autotoxicity in alfalfa has not been positively identified, but is thought to be ethylene and/or medicarpin. The autotoxic chemical is found in higher concentrations in the leaves and flowers than the stems and roots of alfalfa plants.

What affects alfalfa autotoxicity?
The age of the alfalfa stand affects autotoxicity. Stands that are two or more years old will contain more toxins than stands that are one year old or less. In addition, alfalfa plants have a higher level of toxins when flowering compared to alfalfa that is vegetative.

As soon as a stand is killed, the autotoxic chemicals are released into the environment from the decomposing plant material. Once they enter the soil, they will remain there until they break down or are moved out by water. The length of time these toxic chemicals stay in the soil depends on soil type, temperature and rainfall.

On sandy soils, you will see more acute effects of the toxic chemicals, but they will last for less time than on heavier soils. This is because the chemicals will be quickly leached out by rain. On soils with more clay, the toxic chemicals are more strongly attached to the soil particles. This leads to a lower level of damage over a longer amount of time.

How does it affect alfalfa plants?
Both seedling emergence and growth are reduced by alfalfa autotoxicity. Plants that emerge are often stunted and may show purpling, indicating a lack of nutrients. Root growth is most severely affected. Roots are swollen, discoloured, curled and lack root hairs. They are also more branched and less tap-rooted than those of normal alfalfa plants. This negatively impacts the longevity of the stand, as it reduces the alfalfa plants' abilities to take up water and nutrients.

Does it affect alfalfa yields?
Studies have shown yield reductions when alfalfa has been seeded after alfalfa, with no break in between. These yield reductions can be anywhere from 8% to 52% and persist for years. Stands affected by autotoxicity are also slower to regrow after a harvest.

How long do I have to wait before reseeding a field to alfalfa?
If your alfalfa field is two or more years old, you should seed an alternate crop for at least one year after taking it out. This will give the autotoxic chemicals released by the old stand time to decompose and leach out of the soil.

If you have a seeding failure or winterkill, you can successfully reseed that field the same summer or the following spring. The toxins are not present in the first year in new seedlings, meaning you don't have to worry about autotoxicity until the stand is two years old.

What about thickening an older stand?
It is not recommended to try to thicken an old alfalfa stand with alfalfa. While you might get germination and seedling growth at the start, those plants will likely die out over the summer. This is because the size of the autotoxic zone around established alfalfa plants does not leave much space in a field where new seedlings could survive.

Studies have shown that the autotoxic zone is a 16 inch radius from an established alfalfa plant. New seedlings within 8 inches of the established plants often die. Those 8 to 16 inches away survive, but have stunted growth, poor root development and low yields. If you have areas in the field that do not have alfalfa plants, those spots are where it is most likely new plants will survive when seeding into an existing stand.